Method of transporting oil and gas under high pressure in tanks on board a ship

ABSTRACT

The transportation of oil and gas under high pressure in tanks on board a ship is carried out by loading and unloading the oil/gas utilizing a suitable pressurized liquid, for example water, in the individual tanks, whereby during loading, a tank or a group of tanks containing pressurized liquid are filled with oil and gas while the pressurized liquid simultaneously is displaced into the next tank or group of tanks which are to be filled, after which the said next tank/group of tanks are filled and the pressurized liquid displaced into a third tank/group of tanks, etc., and that during unloading, the cargo in one tank or one group of tanks is removed by introducing a pressurized liquid into the tank/group of tanks, and unloading of the cargo in the next tank/group of tanks occurs by transferring the pressurized liquid from the said first tank/group of tanks to the next.

BACKGROUND OF THE INVENTION

The invention relates to a method of transporting oil and gas under highpressure in tanks on board a ship. The invention has been especiallydeveloped in connection with the utilization of so-called marginalfields in the North Sea. An economical utilization of the gas from findsin the North Sea requires that the gas must be recovered and transportedto the consumer by means of a system which does not push the price uptoo high. The immediate solution that comes to mind is transportation inpipelines, but physical limitations such as the Norwegian channel,insufficiant quantity, etc. present hindrance in this regard. One shouldtherefore have other alternatives in view, especially such as may beutilized in connection with smaller finds.

The extent to which an offshore gas and/or oil field can be utilized isamong other things dependent on the economics of the recovery andtransportation system one selects, and the size of the field, i.e., thequantity of the oil and gas which can be recovered, represents animportant parameter. For smaller fields, the situation may be that ifone is unable to find an especially simple and inexpensive system oftransportation, it may not be feasible to utilize the field. Ourdeliberations indicate that storage and transportation of oil/gas underhigh pressure would represent a favourable solution. Such a solution, inbrief, involves taking the entire flow from one or more oil wells onboard ship in high pressure tanks and bringing it to land, where thenecessary treatment, relief of pressure and separation occur. Theland-based plant can serve several fields. Offshore, the field equipmentcan be limited to that which is necessary for connection in order totransfer the oil/gas to a tanker. However, the equipment on board theship must be relatively advanced, especially the navigation equipment,if the system involves locating the well from the tanker. Alternatively,one can naturally utilize a loading buoy.

The invention concerns a method which is to be utilized in connectionwith the transportation of oil and gas under high pressure, and providesa solution to the problems one encounters in loading and unloading.

In accordance with the invention, it is proposed that loading andunloading be carried out utilizing a suitable liquid under pressure,e.g. water, in the individual tanks on board the ship, whereby duringloading, a tank or a group of tanks containing pressurized liquid arefilled with oil while the pressurized liquid simultaneously is displacedinto the next tank or group of tanks to be filled, after which the saidnext tank/group of tanks are filled with cargo while the pressurizedliquid becomes displaced into a third tank/group of tanks, etc, and thatduring unloading, the cargo from one tank or one group of tanks isremoved by introducing a pressurized liquid into the tank/group oftanks, unloading of the cargo in the next tank/group of tanks occuringthrough the transfer of the pressurized liquid from the said firsttank/group of tanks into the next, etc.

The method of the invention can be used both in connection with oil andassociated gas and for gas alone. The pressure in an oil/gas well willusually (at least for a certain period of time) be much higher than 100bar. It is assumed, however, that the most economical solution will beobtained if the pressure, by relieving the pressure through expansion ofvolume, can be reduced to around 100 bar.

With the invention one eliminates the drawbacks associated withpressurized filling of empty tanks, and it also permits unloading to beperformed in a simple manner.

If the cargo is introduced into a non-pressurized tank, an initialpressure drop of about 100 bar over the control valve will result, andthis has the following unfavourable consequences:

Firstly, much of the liquid will evaporate (flash gas) and the volume ofthe tank will be poorly utilized; secondly, the evaporation consumesheat, which causes a drop in temperature and freezing of the wateraccompanying the liquid; and thirdly, with the big drop in pressure, onewill obtain high velocity flow (speed of sound) through the controlvalves, and sand particles accompanying the oil will thus cause muchwear on the fittings and piping. The high flow speed will also producecavitation and noise problems.

To prevent the above, one might use gas as the pressure medium, i.e.,building up the gas pressure in the tanks to about 100 bar by means ofcompressors prior to loading, and forcing the cargo by pressure ontoland by means of the same compressors. However, this would require greatamounts of energy and is unfavourable from the point of view of safetyas compared to pumping water. Relatively little energy is required toincrease the pressure in a water-filled tank from 0 to 100 bar.

The loading and unloading rate is controlled by regulating andcontrolling the amount of water which is removed from or introduced intothe tanks, respectively. The above-mentioned flow control problems willnot be experienced, even at great drops in pressure.

During loading, the drive pressure is supplied "gratis" from the oil/gaswells, and in unloading, the drive pressure is maintained by waterpumps.

The invention will be elucidated in greater detail in the following withreference to the accompanying drawings, wherein

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1, 2 and 3 show a tanker suitable for the high pressuretransportation of oil and gas, in side view, cross section and planview, respectively,

FIG. 4 shows a tanker connected to an offshore installation on the seabed,

FIG. 5 shows a tanker connected to a derrick buoy,

FIG. 6 shows the tanker at the land installation,

FIGS 7a-g are flow diagrams for the tank installation on board thetanker, illustrating the respective conditions under ballast and duringloading and unloading, and

FIG. 8 is a schematic diagram illustrating the phasing in of theunloading of gas so as to hold the oil/gas mixture ratio as constant aspossible during the entire unloading operation.

DETAILED DESCRIPTION OF THE INVENTION

The tanker shown in FIGS. 1, 2 and 3 is provided with a large number ofseparate tanks arranged in groups, with a specified number of tanks ineach group. The tanks 1 are formed as upstanding, relatively slimcylindrical tanks. For example, the tank diameter can be 2 m, while theheight of the tank is 22.5 m. The volume of such a tank will be about 80m³. 100 bar is calculated to be the working pressure.

All of the tanks in one group are loaded/unloaded in parallell. Thereare two sets of main conduits (not shown), such that one group on thestarboard side and one group on the port side can be loaded/unloadedsimultaneously. Rather than in the manner illustrated, the tanks couldalso be arranged horizontally and then be of a length approximatelycorresponding to the length between the forward and after cofferdams.

FIG. 4 shows how the tanker can be coupled to a recovery installationlocated on the sea bed, and

FIG. 5 shows how the tanker can be moored to and connected for loadingof oil/gas at a derrick buoy.

In FIG. 6, the tanker is shown at a land installation.

Of the equipment required offshore, a collecting system in a loadingbuoy or the like, and also water injection equipment, should bementioned. The land installation is a conventional type and contains atreatment plant 2 and a storage installation 3.

The advantages of high pressure loading/unloading are obvious. Oil/gascan be taken directly from the well(s), and when the tanker is notactually on location at the loading site, the recovery installation canbe "put to rest". It can optionally be totally unmanned in theseperiods, or have only a minimal maintenance crew in attendance, forexample. The water injection equipment may for instance be arranged onboard the tanker. The total economy for such recovery of oil/gas istherefore very favourable.

With reference to FIG. 7a, the most important equipment and the mostimportant components in the flow diagram will be defined. Theinstallation is for a high pressure carrier having, e.g., 280 tanks,where the diameter of each tank is 2 m and the height of the tank, 22.5m. Each tank thus has a volume of 70 m³. The total volume will be 19.600m³. The calculated working pressure is 100 bar. The loading/unloadingtime is calculated to be about 16 hours, and per tank/group of tanks aloading/unloading time of 1 hour and 10 minutes is calculated. The tanksare arranged in groups, with ten tanks in each group, or 28 groups oftanks. In the flow diagram, only three of the ten tanks in each groupare illustrated. All of the tanks in one group are loaded/unloaded inparallel. Two sets of main conduits are provided such that one group onthe starboard side and one group on the port side can be loaded/unloadedsimultaneously. The flow diagram shows only one side, for instance thestarboard side. The total loading/unloading capacity is about 1200 m³hour.

There are the following main conduits with branch lines to each group:

loading/unloading conduit

gas pressure line

gas suction line

water supply conduit

water discharge conduit

safety valve conduit

i.e., 12 main lines along the ship, exclusive of various auxiliarysystems.

Each tank 4 is provided with three level sensors, LSL, LSH and LSHH.These sensors can register oil, water and gas. The most important valvesare designated A, B, E, F, G, H and I, respectively. Necessary controlapparatus is provided for the valves, which will be well known to theskilled person.

Each tank has two safety valves with a set point of 105 bar, and apressure switch RS with a somewhat lower set point for alarm andautomatic closure.

A flow control valve is designated FCV-1. This valve controls theloading and unloading and is dimensioned for 700 m³ per hour. Theclosure pressure is 110 bar. The valve is provided with positions and iscontrolled in split range by the flow instrument FIQC-1 and the pressureregulator PIC-1.

FIG. 7a shows the conditions on a ballast voyage. All tanks arenon-pressurized and filled with gas, with the exception of the tanks ingroup 1, which are filled with water up to a level which is sensed bythe sensor LSH.

Before loading begins, the pressure is increased (FIG. 7b) in tank group1 by opening valves A-B and I for this group, and the water pump 5 isstarted. The pump suctions water from a not-illustrated water tank(indicated at the upper right-hand corner of FIG. 7b by the words "fromwater tank"), and the level in the tanks 4 rises while the pillow of gastherein becomes compressed. This can be done at the same time as the oilpressure is built up in the main conduit 6.

When the water pressure in tank group 1 is equal to the pressure in themain conduit 6 for oil, i.e., about 100 bar, and the selector switch atthe control panel has been set on "Loading Auto", the following willoccur, preferably automatically by means of instrument and a computersystem (FIG. 7c). The computer system, in principle a microprocessor,has the possibility of rapid re-programming for differentloading/unloading procedures, adapted to varying parameters in regard totime, composition, pressure etc. The pump 5 stops and the valves E and Hfor group 1 open. Valves A-B-G and I in group 2 also open. The valve Ifor group 1 is closed. The tanks in group 1 are now under pressure fromthe main conduit 6 and the pressure reaches the flow control valveFCV-1.

The flow control valve FCV-1 is controlled primarily by the quantitymonitor FIQC-1, and the set point for the latter is now regulatedgradually from 0 to the desired loading capacity, for example 600 m³ perhour, and loading is underway. If the pressure falls such that there isa risk that the oil might give off gas (for example at 95 bar), thepressure regulator PIC-1 takes over the control of the flow controlvalve FCV-1 such that a constant counterpressure is maintained.

When the loading of tank group 1 on the starboard side has gone on forabout 1/2 hour, loading of tank group 1 on the port side commences.

As the water level in tank group 2 reaches the level sensed by thesensor LSH, this sensor will send a signal to close the valves A. Whenall have been closed, valve G closes and the valves A reassume an openposition. The pressure in tank group 2 begins to increase, the waterlevel rises and the gas becomes compressed. The pressure drop over theflow control valve FCV-1 is reduced and the regulating valve graduallyassumes a fully open position.

When the sensor LSL in tank group 1 signals "oil", valves A and B areclosed, and when all are closed, valves E and H also close. Tank group 1is finished loading and the pressure in tank group 2 will now be about100 bar.

Loading continues as shown in FIG. 7d. At the same time as valves E andH for tank group 1 are closed, corresponding valves for tank group 2 areopened, as well as valves A-B-G and I for tank group 3; the filling oftank group 2 commences and the water is forced by pressure over intotank group 3, while the gas is pressed out and vented off at the mast oris compressed and stored in gas tanks.

If the pressure difference between the main conduit 6 and a tank groupis too great (for example, more than 5 bar), filling will not commenceuntil the water pump 5 has built up the prewsure in the tanks. The waterin the last group of tanks is forced by pressure over into a separatewater tank (not illustrated).

FIG. 7e illustrates the situation during unloading. Before commencing tounload, all the valves E and I are opened, and a counterpressure fromland which is equal to the tank pressure is thus established in the mainconduit 6. When this has been done, and a selector switch at the controlpanel has been set on "Unloading Auto", the following will occur: Thepump 5 starts up and suctions water from the (not illustrated) watertank. When the pressure in the water conduit is equal to the pressure inthe tanks, the valves A and B in tank group 1 are opened and unloadingis underway. The set point for the quantity monitor FIQC-1 is graduallyincreased to the desired unloading capacity, for example, 700 m³ perhour for ten tanks.

The necessary counterpressure to prevent flashing in the system iscontrolled on land.

When the unloading of tank group 1 on the starboard side has proceededfor about 1/2 hour, unloading of tank group 1 on the port side isstarted. In this way, the gas from one tank group is always unloadedsimultaneously with the oil from another group, which gives theadvantage that the gas/oil flow to the land installation will be of anapproximately constant composition for the duration of the unloadingperiod, except at the start/stop of the unloading (FIG. 8).

When the level sensor LSHH indicates "water", the valves A are closed,and when all of these are closed, the valves E and I for tank group 1also close.

The ensuing situation during unloading is shown in FIG. 7f. The valve Hin tank group 1 and valves A and B in tank group 2 open. The pump 5 willnow suction water from tank group 1 and press oil out of tank group 2,and the unloading is still controlled by the flow control valve FCV-1.

When the pressure in tank group 1 has fallen to below 4 bar, the valvesA and F are opened and gas from the gas tanks (not illustrated) isallowed access in order to force the water up to the suction side of thepump, while at the same time the oil line for group 1 is blown empty.

When the level sensor LSL in tank group 1 indicated "gas", valves B areclosed, and when all are closed, valves H and F also close. Gradually,as the tanks in tank group 2 become empty of oil, i.e., when the levelsensor LSHH signals "water", valves A are closed, and when all areclosed, valves E and I also close.

The terminating unloading operation is shown in FIG. 7f. When all thewater has been pumped from tank group 1 and over into tank group 2, thewater is automatically pumped further to tank group 2 and the oil isforced by pressure to the land.

When the pressure in tank group 2 has fallen to below 4 bar, the valvesA and G in tank group 1 are opened, as well as the valves A and F intank group 2, and the compressor 7 starts. The pressure is therebylowered in tank group 1 and is maintained in tank group 2 to raise thewater up to the pump 5. At the same time, the oil conduit to tank group2 is blown empty of oil. When the pressure in tank group 1 has fallen tozero, the valves A and G are closed. In this manner, there will beatmospheric pressure in the tanks 4 when unloading is finished.

"Additional Gas" for refilling the tanks 4 during unloading is takenfrom land or from special pressurized tanks on board the carrier.

We claim:
 1. A method of transporting oil and gas under high pressure intanks on board a ship, wherein loading and unloading are carried oututilizing a suitable liquid under pressure, for example water, in theindividual tanks, said loading comprising filling a tank or group oftanks containing pressurized liquid with oil and gas while thepressurized liquid simultaneously becomes displaced into a next tank orgroup of tanks which are to be filled, after which the said nexttank/group of tanks are filled with cargo while the pressurized liquidbecomes displaced into a third tank/group of tanks, etc., and saidunloading comprises removing the cargo from one tank or one group oftanks by introducing a pressurized liquid into the tank/group of tanks,the unloading of the cargo in the next tank/group of tanks occurringthrough the transfer of the pressurized liquid from the said firsttank/group of tanks to the next, etc.the ratio of the oil/gas mixturebeing held as constant as possible throughout the entire unloadingoperation by phasing in the unloading of gas from the various tanks orgroups of tanks.
 2. The method of claim 1 wherein the steps of loadingand unloading further include detecting signals from sensors located atsaid tank or group of tanks, and automatically controlling movement ofsaid cargo based on said signals.
 3. A method of transporting oil andgas under high pressure in tanks on board a ship, wherein loading andunloading are carried out utilizing a suitable liquid under pressure,for example water, in the individual tanks, said loading comprisingfilling a tank or group of tanks containing pressurized liquid with oiland gas while the pressurized liquid simultaneously becomes displacedinto a next tank or group of tanks which are to be filled, after whichthe pressurized liquid becomes displaced into a third tank/group oftanks, etc., and said unloading comprises removing the cargo from onetank or one group of tanks by introducing a pressurized liquid into thetank/group of tanks, the unloading of the cargo in the next tank/groupof tanks occurring through the transfer of the pressurized liquid fromthe said first tank/group of tanks to the next, etc.,the ratio of theoil/gas mixture being held as constant as possible throughout the entireunloading operation by phasing in the unloading of gas from the varioustanks or groups of tanks, and automatically controlling said loading andunloading.